The present invention relates generally to amplifiers and, more specifically, to an improved amplifier for use in electrocardiographic monitoring equipment.
Electrocardiographic monitoring apparatus generally includes a plurality of electrodes appropriately placed on a patient which transmits ECG signals to an amplifier which are amplified and recorded typically on a strip chart or stored in a memory. The input signal represents the ECG signal as well as noise. Often the presence of a noise does not seriously impair the usefulness of the ECG data, however, noise may saturate the input amplifier and, thus, cause loss of data when the output signal is locked at a fixed value and it takes a finite time for the amplifier to come out of saturation.
One approach for alleviating this problem is to use a narrow bandwidth amplifier. The prior art has generally limited the bandwidth from 1 Hz to 30 Hz. Such a limitation drastically reduces the quality of the signal received and does not provide adequate analysis of the QRS shape, the ST rise and the R-R interval. An effort to remove 60 cycle noise is illustrated in U.S. Pat. No. 4,161,945, to Grossman, wherein a notch filter is inserted between the preamplifier and the power amplifier to remove the 60 cycle signal once it is detected. The control circuit will remove the notch filter when 60 cycle noise is not present.
In the prevention of the transmission of transients, for example, the pacemaker spikes or electrode switching is attenuated in the prior art by forming an integrator or muting stage between the preamplifier and the power amplifier as specifically shown in U.S. Pat. Nos. 3,534,282, to Day and 4,331,158, to Partridge. Manual adjustments of high pass filter, low pass filter and amplifier gain has been provided as illustrated in U.S. Pat. No. 4,261,369, to Allor.
The adjustment of the low frequency 3 dB roll-off point from 0.05 Hz to 1.0 Hz in response to a predetermined level of low frequency noise and the lowering of the upper frequency 3 dB roll-off point from 50 Hz to 10 Hz in response to high frequency noise is specifically described in U.S. Pat. No. 3,569,852, to Berkovits. The low frequency roll-off point adjustment is achieved by changing the time constant of the high pass filter between the preamplifier and the main amplifier by inserting a fixed resistance in parallel with the filter resistance. The upper frequency roll-off point is modified by inserting a capacitor as a filtering element. Both of these adjustments are activated only at a predetermined level of noise to modify the roll-off point to a single preselected level and are maintained as long as the noise is above the predetermined level. Since only a single adjustment is available in the Berkovits patent, a considerable amount of instability, distortion and noise can be introduced until the threshold of the comparators are exceeded.
Thus, it can be seen that an improved electrocardiographic amplifier having a frequency response range which is a direct function of noise and can be continuously adjusted is needed.